Abstract
A method to improve radioactive waste drum activity estimation in Segmented Gamma Scanning (SGS) systems was developed for homogenous content. We describe a method to quantify the activity of spatially distributed gamma-emitting isotopes (‘hot spots’) in homogenous content waste drums without the use of a collimator. Instead of averaging all the detector's readings we treat it as many different spatial samples as if we have multiple detectors surrounding the waste drum ("virtual detectors"). From these readings, we form a general linear model. Next, we derive the Maximum Likelihood Estimator (MLE) for the multiple sources position and activity. We solve this hyper-dimensional search problem using an Alternating Projections (AP) technique which transforms the problem into a simpler one-dimensional maximization problem. We tested this method using a mathematical simulation with a various number of sources, at random activities and positions for several energy bands. The preliminary results are consistent and show large improvement of the accuracy with comparison to industrial SGS systems and the same accuracy as new methods which exploits the spatial samples. Furthermore, since this method eliminates the need for heavy led collimator, none of the sources is blocked for the whole measurement period, which provides increased count rates and decreases the total measurement time.
Highlights
G HIS Gamma-ray spectrometry uses the fact that each isotope emission gamma-rays in proportion to the mass of that isotope in the sample
We describe a method to quantify the activity of spatially distributed gamma-emitting isotopes (‘hot spots’)
After finding the first source position ݏԦଵ, we search for the second source using [7], where the attenuation matrix A is built from two attenuation vectors – the first vector is from the position we found in the previous search, ܽԦ൫ݏԦመଵ൯, and we search over the possible positions of the second source attenuation vector
Summary
G HIS Gamma-ray spectrometry uses the fact that each isotope emission gamma-rays in proportion to the mass of that isotope in the sample. Not all emitted radiation will be detected by the detector. This is caused due to absorption and dispersion of the photons in the materials in its path to the detector, and in the source itself (self-absorption). Another effect is the energy efficiency of the detector. In order to estimate the matrix density, many systems use a preliminary step of using an RTR system (real-time radiography) with X-ray, known external gamma or neutron source
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